Class ThreadLocal<T>

This class provides thread-local variables. These variables differ from
their normal counterparts in that each thread that accesses one (via its
get or set method) has its own, independently initialized
copy of the variable. ThreadLocal instances are typically private
static fields in classes that wish to associate state with a thread (e.g.,
a user ID or Transaction ID).

For example, the class below generates unique identifiers local to each
thread.
A thread's id is assigned the first time it invokes ThreadId.get()
and remains unchanged on subsequent calls.

Each thread holds an implicit reference to its copy of a thread-local
variable as long as the thread is alive and the ThreadLocal
instance is accessible; after a thread goes away, all of its copies of
thread-local instances are subject to garbage collection (unless other
references to these copies exist).

Constructor Detail

ThreadLocal

Method Detail

initialValue

Returns the current thread's "initial value" for this
thread-local variable. This method will be invoked the first
time a thread accesses the variable with the get()
method, unless the thread previously invoked the set(T)
method, in which case the initialValue method will not
be invoked for the thread. Normally, this method is invoked at
most once per thread, but it may be invoked again in case of
subsequent invocations of remove() followed by get().

This implementation simply returns null; if the
programmer desires thread-local variables to have an initial
value other than null, ThreadLocal must be
subclassed, and this method overridden. Typically, an
anonymous inner class will be used.

get

Returns the value in the current thread's copy of this
thread-local variable. If the variable has no value for the
current thread, it is first initialized to the value returned
by an invocation of the initialValue() method.

Returns:

the current thread's value of this thread-local

set

Sets the current thread's copy of this thread-local variable
to the specified value. Most subclasses will have no need to
override this method, relying solely on the initialValue()
method to set the values of thread-locals.

Parameters:

value - the value to be stored in the current thread's copy of
this thread-local.

remove

public void remove()

Removes the current thread's value for this thread-local
variable. If this thread-local variable is subsequently
read by the current thread, its value will be
reinitialized by invoking its initialValue() method,
unless its value is set by the current thread
in the interim. This may result in multiple invocations of the
initialValue method in the current thread.